1. Field of the Invention
The present invention is a covert laser retroreflector system of gating open a coded laser retroreflector in an interrogated friendly target by a laser interrogator having a dedicated receiver therein in which specifically coded radiation transmitted from the interrogator triggers the retroreflector to reflect back another coded signal to the dedicated receiver to identify the target as being friendly.
2. Description of the Prior Art
At the present time there is no known wide field-of-view (FOV) laser retroreflector which can be interrogated successfully and yet remain covert. Prior systems require a laser interrogator and a return signal from the target at some other wavelength, such as millimeter or microwave return. These systems require extra antennas which are omni-directional and whose radiation can be detected by opposing sides. Another approach includes a laser signal return, but necessitates that the on-board laser would fire a second return after some delay. Two lasers, one for the interrogator and another for the transponder would require a more complex laser system.
A laser interrogator presents no technical problem in the system of the present invention. The purpose of the present invention is to provide a covert laser retroreflector system with a wide FOV for cooperative battlefield identification of frield-or-foe (CBIFF). The problem is to provide a means of modulating a retroreflector such that the retroreflector is operational, i.e. reflecting, only when desired, yet is non-operational if illuminated by an enemy laser beam. The problem is solved by gating open a retroreflector at a specific time for a short time interval and modulating the laser pulse radiation in the form of a preassigned code. By controlling the transmission of the retroflector in this manner, the retroreflector may be undetectable and unuseable by non-friendly forces, such as enemy laser designators or laser homing missiles.
Various target mounted receivers, or reply links, for laser interrogators have been considered. Some of these reply links that have been considered are as follows. One reply link uses a longitudinally excited Stark Cell modulator retroreflector for use at carbon dioxide (CO.sub.2) laser wavelengths of 10.78 .mu.m. The Stark modulator provides a 25% depth of modulation which is unacceptable for CBIFF operation. Further, its operational lifetime of only two weeks is insufficient for this application. Another consideration is a mechanical shutter retroreflector in which controlled modulation of a retroreflector can be obtained by placing a mechanical shutter in front of the retroreflector. A large FOV is possible with the mechanical shutter approach but has insufficient bandwidth for the CBIFF application.
Another system approach involves a two-way laser system in which the reply signal is provided with a second laser on the friendly target with its laser beam modulated. This approach has a more complicated design since a second laser is required, and the direction of the laser interrogator must be detected by the receiver on the target before responding to the interrogation so that the target laser may be pointed in the correct direction. Another system that is somewhat complicated involves a reply signal to the laser interrogator that uses a millimeter wave/microwave transponder on the friendly target. This approach requires transmitting antennas that are omni-directional and therefore can be detected by non-friendly forces by proper design of their receivers.